Fix Metal MSAA in Viewer.

Sets the sample count so we create the MSAA color buffer, and
sets the manual MSAA resolve flag for the created rendertarget.

Change-Id: I577bc619ce0ce8490704135c72139ad7a05904e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/407818
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
2 files changed